PP1501 - Changes in the Determination of the Result of the VEMP by Correction of the Muscle Contraction Power

Abstract:
The cervical vestibular evoked myogenic potential (cVEMP) is one of the tests that evaluate vestibular function. The degree of contraction of the SCM muscle during the test is known to affect the value measured in the cVEMP test. The amplitude difference between P1 and N1 (non-rectified interamplitude) was measured. The correction of the non-rectified interamplitude (rectified interamplitude) was obtained using the contraction power of the sternocleidomastoid (SCM) muscle. When the original measured value was corrected, considering the SCM muscle's contraction power, the determination (right hypofunction, left hypofunction, and normal) of the test result was changed by about 24%.

Summary:
Rationale
The cervical vestibular evoked myogenic potential (cVEMP) test reflects the function of the saccular macula, inferior vestibular nerve, and vestibulospinal tract. cVEMP is measured in contracted ipsilateral sternocleidomastoid (SCM) muscle with an inhibitory myogenic potential by ipsilateral saccular stimulation. Therefore, it is known that the degree of contraction of the SCM muscle during the test affects the value measured in the cVEMP test.

Purpose
The purpose of this study was to find out the effect of correcting the values measured by the cVEMP test on the determination of the results of the cVEMP test in consideration of the tone of the SCM muscle. So, when the contraction power of the SCM muscle was measured, and the difference between the amplitudes of P1 and N1 was corrected using that value, the frequency of changes in the test results determination (right hypofunction, left hypofunction, and normal) was investigated.

Methods
Data were analyzed from 115 patients who underwent the cVEMP test from May 2019 to July 2022. During the cVEMP test, they wore insert earphones, and the stimulus was 500Hz and 90dB nHL of tone burst sound. The latency of P1 and N1 and the amplitude difference between P1 and N1 (non-rectified interamplitude) were measured. At that time, the degree of contraction of the SCM muscle was measured, and the corrected interamplitude of the amplitude difference between P1 and N1 (rectified interamplitude) was obtained using that value. The evaluation of vestibular function (interaural amplitude difference ratio, IADR) was calculated as (Left interamplitude – Right interamplitude) x 100 / (Left interamplitude + Right interamplitude) (%), when the absolute value of the calculated value exceeded 35%, it was determined as a unilateral vestibular hypofunction.

Results

When the non-rectified interamplitude and rectified interamplitude values were compared with the paired t-test, the two values showed no statistically significant difference (p-value = 0.729).

However, when the patterns of result determination before and after rectification were reviewed, In 91 patients with normal range in non-rectified IADR, there were 5 patients with right vestibular hypofunction, 77 patients with normal range, and 9 patients with left hypofunction in rectified IADR. Six subjects with right vestibular hypofunction in non-rectified IADR showed right hypofunction in 3 subjects and normal range in 3 subjects in rectified IADR. In addition, 18 subjects with left vestibular hypofunction in non-rectified IADR showed left hypofunction in 8 subjects and normal range in 10 subjects in rectified IADR. Therefore, when the original measured value was corrected, considering the SCM muscle's contraction power, the determination of the test result was changed by about 24%.

Conclusions
In the cVEMP test, when the measured value of cVEMP was corrected in consideration of the tone of the SCM muscle, the result determination was changed in about a quarter. Therefore, it can be considered that rectifying cVEMP measurements in this way is helpful in more accurately evaluating the functions of the saccule, inferior vestibular nerve, and vestibulospinal tract.